1. Field of the Invention
The present invention is related to an automatic depth compensating device and the incorporation of the automatic depth compensating device as an improvement in a rotary drill and more particularly to an automatic depth compensating device which has first and second axially aligned pistons positioned in first and second bores wherein the area relationships between the surface areas of the first and second pistons and the bore is such that the pressure in the environment and the pressure in an adjacent volume act on the piston to maintain the pressure in the volume proportional to and greater than the pressure in the adjacent environment so that lubricant in the volume is forced into the environment around a seal therebetween. Furthermore, the present invention is more particularly directed to a rotary drill including the improvement of this type of automatic depth compensating device.
2. Description of the Prior Art
In all rotary drills applied to the drilling of wells in the ground as well as many types of devices used in wells which are either drilled or in the process of being drilled, there is a need to protect the inner working parts of the drills and devices from pressure and from the environment in which the drill device is working as the environment is often very agressive or damaging. The need for protection is most critical at points where there is relative movement between different parts of the device. Typically, problems of this type occur in the movement of drilling cones of tricone drill bits and in the movement of a single cutting head relative to drill bit body as in a Barnetche drill bit, such as that shown in U.S. Pat. No. 4,154,312. The problem with regard to protection also exists in devices other than drill bits as, for example, in downhole motors, downhole pumps, etc.
In devices such as those discussed above, since there is movement between parts, there is also a need for bearings of some type. The bearings in turn require a clean self-contained lubrication system and the fact that the relative movement occurs between different parts of these devices creates a need for a seal between the separately moving parts which will isolate the internal lubrication system from the downhole environment. It is these seals which are usually the most vulnerable element in these devices.
In actual drilling there are a number of adverse factors which exist simultaneously. Furthermore, many of these conditions exist in the downhole environment even after drilling is completed and, thus, have an impact on other types of downhole devices as well as drills.
One factor contributing to the performance requirement for seals on downhole devices is pressure. Due to the need for circulation of drilling fluids which are commonly employed in rotary drilling of high pressure wells, the pressure on devices at the bottom of the well are extreme. If the internal pressure of the self-contained lubrication system of the downhole device were to be at atmospheric pressure, the seals on the devices would be required to withstand tremendous differential pressures since the downhole pressure of the environment is very high. Further, the environment at the bottom of the hole is extremely agressive or damaging due to the presence of drilling fluid, mud, additives, drill cuttings, etc. The environment, thus, will produce a very erosive action on the seals. Failure of the seal under the large differential pressure load will result in leakage of the agressive downhole elements through the seal into the device itself producing rapid deterioration and failure of the internal components such as the bearings.
Other factors which also contribute to the pressure and agressive environment are vibration, load-shock, high temperatures and the particular formation being drilled.
In order to provide compensation for the extreme downhole pressures and to reduce the damaging effect of the agressive environment on the seals, pressure compensators have been used. An example of this can be found in U.S. Pat. No. 4,154,312 which is directed to the Barnetche bit. In the pressure compensator disclosed in this patent, a diaphragm is positioned between the interior and exterior of the downhole device in order to equalize pressure between the volume inside of the drill bit and the environment. Thus, this type of pressure compensator reduces the pressure differential to zero. However, pressure compensators such as that disclosed in the Barnetche bit patent have the disadvantage that at best they produce equal pressure between the inside of the drill bit and the environment. Furthermore, generally due to hysterisis and viscosity of internal lubricants the internal pressure is less than the external pressure. This means that at the seal, in the case of the differential pressure being zero, the seal tends to dry out since no lubricant is added to the seal faces. In the case of negative internal pressure, even though it is small, there will be a small amount of leakage in which the agressive external environment will enter between the sealed faces thereby accelerating seal wear. Furthermore, the harmful elements of this agressive environment will also have a damaging effect on the internal parts of the drill bit such as the bearings.
Other attempts have been made to solve this problem by variations of the basic compensator. One variation is including compensating lubricant bags or bellows. This has been utilized where the device interior volume is sufficiently high. The problem however, with this type of compensator is to find a source of motive force. One source which has been used is the pressure of the injected drilling mud, passing down the drill stem and through the drill bit, through a diaphragm device on the tool so as to maintain the internal lubricant permanently at an increased pressure. The problem with this solution is that it does not give true control of the differential pressure across the seal since there is no direct relation between mud pressure and downhole exterior pressure. The end result of this type of technique is that there is too little or too much lubricant flowing across the seal.